Radio navigational system



Sept. 12, 1950 c. w. EARP ET AL 2,521,702

RADIO NAVIGATIONAL SYSTEM Filed Aug.. 4, 1945 4 Sheets-Sheet l A llorney Sept 12, 1950 c. w. EARP ET A1. 2,521,702

RADIO NAVIGATIONAL SYSTEM Filed Aug. 4, 1945 4 Sheets-Sheet 2 Attorney sept. 12, 195o Filed Aug. 4, 1945 C. W. EARP ET AL RADIO NAVIGATIONAL SYSTM 4 Sheets-Sheet 5 MWF @wif

Inventor GXNRLES, Wmmqm. Emil LwmuasA Eme. Stamm..

` A Harney Sept. 12, 1950 Filed Aug. 4, 1945 C. W. EARP ET AL RADIO NAVIGATIONAL SYSTEM 4 Sheets-Sheet 4 @sumas mi'. STK@ NLS d Make-s.

n A Horn@ Patented Sept. l2, 1950 RADIO NAVIGATIONAL `SYSTEM `Charles William Earp and Charles Eric Strong,

London, England, assignors, by. mesne assignments, to International Standard Electric Corporation, New York, N. .Y., a corporation of Delaware Application August 4, 1945, Serial No. 608,956 In Great Britain August 4, 1944 7 Claims. l

The present invention relates to radio navigational systems. n Radio navigational systems are used amongst other purposes for guiding a mobile radi'oreceiver of suitable type along a specified pathLor example a glide path. or approach Pathy or for enabling a suitable radio receiver to obtain its bearing with respect to a transmitting beacon or to obtain the direction of propagation of the received electromagnetic waves. Hitherto radio navigational systems have used radiations which are distinguished or characterised by amplitude modulations. Systems of this type suffer from one great disadvantage, namely the presence of interference in the receiver due to other amplitude modulated transmissions. Such interference cannot be eliminated except by theuse of different frequencies for different beacons, but even so there may be similar interference due to other radiating systems. This type of interference is the same as the well-known heterodyne interference obtained between broadcast entertainment signals of the amplitudel modulated type when two or more transmitting stations operate on the same or adjacent irequenciesl It is now well established tll'lat'a'number"i-|l broadcast transmitters utilising phase or frequency modulation can operate onthe sam'ecarrier frequency and each dominate in its lown service area.

The heterodyne interference in then.

lpja'rison wave. Thesweep is the cyclicV variation lwhichthe phase modulation is eiiected atvthe receiver, namely an automatic direction vfinder 'or radiccompass arrangement. Figure 2 shows in block schematic a more detail arrangement ofthe receiver of an automatic radio vdirection finder embodying theinvention.

Figure 3v shows in block schematic a detailarrangement of vanitem of Figure land an alternative to a portion. oi ythe arrangement shown in Figure` 2.

I'Figure y4 illustrates one form 'of antenna comv mutating arrangement;

Figur'eigis an embodiment of the invention in which rthe `phase modulation is effectedA at `the "transmitterjn'amely a Vblind approach path bea- 'v Figure 6 showsI in block schematic a receiver for service area is eliminated entirely by the use of a receiver amplitude limiter. Thus in such'a system which is able to transmit intelligence while maintaining the amplitude of the radiated waves constant, an amplitude limiter 'may be used in the receiver to eliminate any interference byf' a Weak signal.

It is an object of this invention to apply the principles of frequency or phase modulation reception to radio navigational systems so as to gain for such systems all the advantages of irequency or phase modulation reception and 'thereby enable a plurality of radio beacons to utilise the same transmission frequency.

It will be observed that as regards radio navigational systems it is not possible to radiate `frequency or phase modulation directly since the j plane.

" use withVV ay beacon of the type shown in Figure '5;

` Figure '7 comprises various curves used in the description of Figure 6;

Figure s'shows in block schematic form anirtherembodiment of the invention in a transmitter -for dening a precise course for aircraft in a horizontal plane or glide path ina vertical Figure 9 comprises varicuscurves used` inl the vdescription -of Figure 8;

Figure 10 illustrates a further embodiment-0f the invention in an omni-directional beacon, and Figure 11 shows in block schematic'form a receiver for use with a beacon 'of the type shown in Figure 10 for obtaining bearing indications oi 'l vention wherein the frequency or phase'rnodulation islfeiiected at the receiver for determining lthe direction of propagation of the received electromagn'etic waves and consequently the bearing of a receiver with respect to the transmitting beacon from which the received waves emanate.

The 'rceiver may of course be mobile and the ftransmitter `stationau'y or the receiver may be' stationary' and the transmitter mobile, for 4eX- ,.,ampla `carriedfby an aircraft, or both receiver 3 and transmitter may be mobile, as in the case of two aircraft or two ships.

In the example to be described the receiver antenna system comprises a plurality of antenna arranged in a circle, Three antennae AI A2, A3 are shown by way of illustration but any greater number may be provided. It is assumed the transmitting beacon radiates waves of constant frequency and phase, These antennae are Vconnected cyclically and successively to the radio receiver by means which will be described presently, and thus the signal fed to the receiver comprises a wave whose phase is suddenly changed at the change of antenna connection to the receiver, and the magnitude of the phase change depends upon the position in the antenna system of the antenna connected at the moment to the receiver with respect to the direction of propagation of the received waves.

The receiver is illustrated as comprising anamplier represented by block l, an-amplitude limiter .represented by .block k2 and a phase or frequency .,discriminator represented by block 3 and a filter circuit represented by block 4. Aphase comparilson instrument `and indicator is represented by block 5. Block represents a source of oscillations which controls, as will be seen hereinafter the commutation of the antennae AI, A2, and A3 and is therefore convenient as the source of a vcomparison wave, which is fed to the phase com- `parer 5 together with the output from lter 4.

Block T represents a device for producing time phased pulses from the oscillations from `l, there being, per cycle of the oscillations from 6, a pulse for each antenna Al-A3. In an alternative arrangement, 1 may be a pulse generator and 6 an arrangement for producing the appropriate sub- Aharmonic of the pulse frequency.

The pulses in the output of 1 are preferably pulses of rectangular waveform.

The .arrangements for commutating the antennae Al-A3 or switching .them cyclically and successively to ,connect them to the amplifier I of .the receiver, comprise yrespective detectors DI, D2, D3 which are normally non-conducting and are connected in series with their associated antennae as lshown in more detail in Figure 4. The .pulses from i relatively phased. with respect to each other are applied to the detectors DI D2, D3 so as to make them conducting for the duration of the Apulse and hence connect the yantennae in turn ,to the amplier i.

In the case of a simple homing device only two aerials are necessary. When automatic direction finding is required, three or more aerials are remquired, when they will preferably be arranged at equal intervals around the circumference of a circle.

The Ainput to amplifier l is therefore a succession of closely but equally spaced'pulses of the frequency of the received waves, the pulses having approximately equal amplitudes. The pulses of received waves are required to be of 4equal amplitude, lbut owing to practical imperfections such as unequal aerials, detectors or pulses from `'l---or owing to interaction between aerials, they are not lexactly so. The received Wave train is therefore subjected to the action of amplitude limiter 2.

Output from 2 is of constant amplitude, but the `phase of the waves is subjected to sudden changes at the insta-nts of changeover from one aerial to Vthe next, the amounts of `phase change depending :upon aerial spacing, and .the direction of arrival ofthe received waves.

The phase discriminator 3, is identical to any .3 therefore contains this frequency, which is filtered by filter i to produce a sine-wave. Phase comparison with oscillation wave from 64 serves to indicate the direction of propagation of the received waves in the following manner.

In the case where two aerials, only, are used, phase modulation of the received waves is inverted in sense according to which aerial is the more advanced in the direction of propagation o f the received waves. When the two aerials are broadside to the wave-front, no frequency or phase modulatio-n is produced. Output from the receiver filter 4 is arranged to be in-phase or anti-phase with output from 6, and the phase comparer 5 may then be a simple left-rightindicator, or dynamometer course meter.

Thus the arrangement illustrated in Figure 1 utilising two iixed aerials on an aircraftmay be used as a homing `arrangement for the aircraft, by turning the aircraft so that the two aerials are always broadside to the Wave front received, which is indicated by the meter 5 reading Zero. Alternatively, utilising two aerials rotatable as a system, the arrangements illustrated in Figurel may be used for .determining the bearing of the aircraft carrying the receiver, .by rotating the aerial system until the meter 5 indicates zero, the bearing being indicated by means of a compass scale associated with the rotatable aerial system in known manner.

When three vor m'ore aerials are used equally spaced around a circle, the phase ofthe phase modulation sweep of the received Waves produced by the commutation of the aerials varies directly according to the direction of propagation of the received waves with respect to the aerial system. It is then desirable that thediscriminator ordemodulator 3 gives an output Whose amplitude varies linearly with phase change of the input waves, in order to Aobtain a bearing indication without .error on the phase indicator 5.

When using a phase demodulator of a phase modulation communication system, output from the discriminator due to a phase transient is proportional to the sine of the phase angle, so that for adjacent-antenna spacing of more than electrical degrees (that is M5 approx. A -operating wave length), there is some distortion of demodulation. The effect of this distortion in the present use of determining bearing is to produce an error which is lequivalent to the well-known octan'tal error of the `orthodox Adcock Directio-n Finding System. When four aerials are used in the present case, the error is octantal, and in order that this error shall not be serious, adjacent aerials should not be spaced Vby more than about 5 A suitable form of discriminator arrangement which avoids this limitation on aerial spacing utilising more than four aerials will now be described in relation to Figure 2 of the accompanying drawings, which shows the arrangement of Figure 1 in more detail as regards the discriminator 3. The blocks are given the sam-e designations as in Figure l, the discriminator arrangement being shown in block schematic from' within the broken line rectangle 3 and the antenna system and switching or commutation arrangement being collectively indicated by block 8, the outputs from ve aerials being represented and connected to a common point. By this means the aerial spacing can be increased beyond A/ only because the differential delay of the discriminator paths equals the time of operation of one aerial. As will be seen hereinafter, the differential delay output of the two paths contains phase steps which are differential adjacent phase steps of the original modulation.

Block 9 represents a constant frequency oscillation generator of known type, for example a crystal controlled generator, giving in its output frequency F. Block I0 represents a modulator or detector stage to which is fed the phase modulated wave from' the output of the amplitude limiter 2 at frequency f. The stage IIJ gives in its output the frequencies (f-l-F) and (f-F) which are fed to filter represented by block Il and which passes either frequency (f4-F) or (-F) to a second modulator or detector stage represented by block I 2, which is also fed directly with frequency f from the amplitude limiter 2. The second modulator or detector l2 gives in its output sum and difference frequencies among which is either (f-F)-f=-F when filter II passes (f-F) or (f-I-F) -f=F when filter I i passes (f-I-F). Frequency F in the output of the second modulator I2 is selected and passed by filter represented by block I3.

It should be noted that filter II must select one sideband only, either (f-Fl or (f-I-F), yfor 1;.;

will cause mutual interference and deep amplitude modulation.

The output at frequency F from filter I3 is fed to the frequency discriminator or phase demodulator represented by block I4 which yields a low frequency wave corresponding to the frequency or phase modulation of the output from the amplitude limiter 2.

It will be observed that the waves in the output of the amplitude limiter 2 are divided into two portions, one fed directly to the second modulator I2 and the other via the first modulator and filter IIJ, I I to the second modulator I2. vThese two paths have a differential delay or transit time. The frequency is displaced in the path l El-I I by a constant amount and recombination of the waves from the two paths in the modulator or detector stage I2 provides in the output thereof a phase-modulated Wave, (modulation 4being the differential modulation of signals from the two paths), of constant mean frequency which is demodulated by the normal type of phase demodulator or frequency discriminator I4.

Preferred practical forms of items 6 and l, of Fig. l are the following:

6 is a stable source of oscillation, for example, a crystal controlled oscillator, the output of which is fed to item 1 which may be a multivibrator type circuit adapted to produce pulses of rectangular wave form of the required duration of an antenna commutating pulse. The said 6 pulses are passedthrough a Vpassive delay network or artificial line represented by block .l5 Figure 2 from which tappingpointsDI-D provide the necessary system of time phased pulses for antenna commutation. This arrange-A ment avoids the use of a plurality of electronic devices and provides the necessary pulses at Another very suitable form of commutating arrangement for the commutation of `the aerials is shown in block schematic inl Figure.` This arrangement will be described on the assumption that there are eight aerials AI-AB having corresponding series connected crystal detectors DI-D8.

Referring to Figure 3, the block I1 represents a square wave oscillation generator, for example of the multivibrator type of circuit, or `a-sinewave oscillator followed'by a squaring arrangement, for example an amplier limiter. /To take a concrete example, the generator l1 produces waves having a frequency of'33.3 kilocycles per sec. The output of I1 is fed toa differentiating circuit represented by blockA I8 and the negative pulses of the differentiationproduct-are eliminated in known manner, for example, by means of a rectifier, thus producing a train of short positive pulses at 30 micro-second intervals which is fed to a'frequencydivider circuit represented by block I9. The frequency divider may comprise any well known multivibrator `circuit and the train of positive'pulses from IB is applied to synchronise the multivibrator circuit on the eighth subharmonic (there being eight aerials) of the repetition frequency of the pulses,

namely l kcs.=4.l6 kos.

and pulses are derived in the output of I 9 at this repetition frequency, having a spacing period of 240 microseconds. The pulse train in the output of I9 is applied to trigger the first of a series of eight Kipp relays, that is multivibrator circuits arranged to have one stable and yone un.,- stable condition and which define time periods of 30 microseconds. That is to say, after being triggered from its stable to its unstable condition, the relay automatically returns to its stable condition at the end of 30 microseconds. These relays are represented in Figure 3 by the blocks KI K8, there being one for each aerial AI-A8. A pulse, obtained from one relay is applied to trigger the next in the series as indicated by the series or cascade connection of the blocks KI-KB. Y

As described up to this point, the 30 p-second intervals defined by the Kipp relays KI-KB are not exactly equal, as they depend upon the individual adjustments of each relay. In order to stabilise these 30 ,LL-sec. intervals, the pulse train of 30 ,1i-sec. intervals developed by the dierentiator I8 is introduced to all of the relays KI-KB in such sense as to encourage triggering towards their stable states..l Initially, the na- 2; 52.1; 'zoe tural periods of. the relays are adjusted to be rather greater than 30Y ,rr-sec. (say 35-40 n sec;), and the 30 n-sec. train is' used to accelerate and control the exact instants of return to stable equilibrium. As only one relay can be undergoing an active cycle of operation at one time, only one relay is returned to stable equilibrium one pulse. The action of return triggering of this relay produces the start triggering of the next relay, which is in turn restored to equilibrium by the next pulse.

In a preferred arrangement the relays each comprise a single multi-grid valve whose electrodes are inter-connected as a Kipp relay.

The square wave form of the output of the relays, it will be observed has an on-oi ratio of 1:7, and the commencementA of the off period oi one coincides exactly with the commencement of the on period of' the next successive one as indicated at 201-208.

The pulses 201-2023 are: applied at suitable energy level to the respective crystal detectors 'Dl-D8, thereby switching the associated antennae AI-AB onto the receiver in succession` for periods of 30 microseconds each, every 240' microseconds. Thus a cyclical switching of the aerials is achieved at an overall frequency of 4.16 kcs;

The aerials Al-AB are in general at varying path lengths from4 the transmitting beacon and thus a sudden change in'phase of the radio frequency carrier wave isachieved when one antenna is switched on" coincident with the switching on of the next antenna in order;

The output of the frequency divider l is used as the comparison wave and may be passed through a filter to produce a sine waveform or undergo other treatment depending upon the type of phase comparer 5 employed.

The most practical form of detector for the ,A

antenna switching detectors D1 D2 D3 etc. depends upon the frequency band on which the system is to operate.

On very short wavelengths, it is necessary to provide a signal path of very low impedance during the pulse, but it must not provide a path due to self capacity when the pulse is not present. For this reason, it has been found that the detecting crystal oiiers the most practical solution. Typical-iigures for a suitable crystal for antenna commutation on frequencies of about 100 mc./ sec.

are:

Resistance 200 ohms (positive bias) and 2000 ohms (negative bias). Capacity 1ML-:1500 ohms reactance.

A crystal detector is connected between each aerial and its associated transmission line and alsobetween each transmission line and its junction to other lines. By this means and the use of some resistance shunt across each line the eifective on-ofi (amplitude) ratio of the received waves from each aerial is much. improved, and the receiver is always connected. via a single ransmission line to a single antenna, avoiding loading of the receiver input by a number of lines and possible short-circuit at some frequencies.

On longer wave-lengths, diode detectors will be found most suitable as Dil-D8 for antenna commutation as the higher self-capacity (for a given conductivity) provides only a very small leakage path, and the resistive leakage is much less than that of the crystal detector.

The method of connection of the antenna switching detectors should preferably be such that when the detector is non-conducting, the

8, aerial itself is'not` only4 disconnected from the receiver, but also prevented from reflecting signals, with resultant inter-action with anotherl of the aerials which is being used. One practical form of aerial and switching detector is shown as Fig. 4. This aerial is illustrated as a folded monopole A which has a vertical height of M4 for the preferred workingV frequency. When the detector D is non-conducting, the aerial is disconnected from the line, and left as an opencircuited quarter wave line whichv is incapable of absorbing or reiecting the signal wave. 2l is the transmission line connecting the aerial A to the common junction J and is shown. as a coaxial line. A condenser 22 is inserted between the crystal detector D and the line 2|-l The appropriate time phase pulse from the phased pulse producer l Fig. l is applied across the 'crystal detector D- by the lead 23 through a resistance or choke 24 as shown.

Ultra high frequency direction finding system In a direction nder which is to operate over a frequency range, of, say, mc./sec. to 150 rnc/sec. it is quite `practical to use a group-of aerials arranged in a circle of diameter A or even 2k, when it 1oecomes necessary to use aV total of about 8 aerials, and about 12 aerials, respectively, When the effect of obstacles at the receiving site is far less error-producing than` for the normal Adcock system. i

At such frequencies, thepass-band of a normal receiver is about 100 kc./sec. which imposes a time of response of about 1.0M sec. A-practical time of operation of each aerial corresponding to this response time is 30 ,a sec. So that for the 8 antenna system, the cyclic period isy 240 ,u sec. and cyclic frequency of modulation a little over 4 kc./sec. Such a frequency of commutation is very convenient, as commutation produces a negligible interference with reception of speech signals, so that station identification and traflic working can be achieved simultaneously with direction nding.

Long wave direction finding system The arrangements hereinbefore described provide a much-improved automatic direction finder system for use with long waves, particularly in cases Where antenna space is limited, as on a mobile vehicle. The normal Adcock system is unsuitable when antenna spacing is a very small fraction of the operating wavelength, as the differential signal from two accidentally unequal antennae is no longer a measure of phase difference. In the arrangements according to the present invention, no such limitation is imposed, for unequal signals are rendered equal by the action of the receiver limiter 2. v

Speed of antenna commutation must, of course, be much reduced, as compared with ultra high frequency working, but the reduced commutation speed is still consistent with the accepted standards of receiver selectivity for such frequencies.

In a long wave system, the antenna commutating diodes D will normally be connected in series with aerials A which are tuned in turn by the same reactive network. In practice, since all aerials are ultimately connected by the same line from the junction J to the receiver amplifier l, each aerial A is connected through a diode D of low impedance to the matching network or iirst tuned circuit of the receiver, so thatf each aerial is tuned by thek samel network. Hence, tuning error does notiproducerrelative phasedisplacement of signals from the various aerials, and no direction nding error can arise.

Radio navigational beaconsgeneral It will be understood by those skilled in the art, that the antenna commutated beacon is identical in principle to the antenna commutated direction nding system. When a transmitter is commutated to a plurality of aerials, the signal received at any location in space bears the same modulation that would appear in the receiver if receiver and transmitter were to be interchanged in position. Thus, the automatic direction finding system already described is readily converted to an omnidirectional beacon by substituting a transmitter for the receiver. It is, however, necessary to transmit a reference signal for phase comparison with the modulation of the received signal.

There is one other feature in which beacon and direction finder systems are liable to differ. In the case of direction finding it will rarely, if ever, be of advantage to connect the receiver to aerials through transmission lines of different electrical lengths. In the case of the beacon, however, it may be of advantage to use different lengths of line in order to set up a course line which is not exactly at right angles to a broadside array of aerials. For example, in setting up a glide-path for aircraft, it is not necessary to make a broadside array of aerials inclined at a very small angle from the vertical: it is more practical to place all aerials at ground level and to phase them in such manner that no phase modulation occurs along the length of a glide-path of very small inclination to the horizontal.

The same precautions are necessary for avoiding interaction between aerials, as for the direction nding system. When aerials are disconnected from the transmitter, they must be incapable of absorbing or reflecting energy from other aerials.

The crystal detector is, of course, not capable of handling large powers, so another technique becomes necessary. Suitable diode switching detectors could be made, though large currents must be passed, and further, inability to handle the complete power would render them liable to generate harmonics of the transmittenand to radiate them. A slight practical disadvantage is that a rather large power is required for the various switching pulses. Despite these factors, it is, however, reasonably practical to use switch-v ing diodes, particularly if they may be connected in pairs back to back in the signal paths to the aerials, permitting a substantial saving of pulse power, and a substantial reduction of necessary diode emission. It is, of course, possible to use a separate amplifier, which is easily pulse modulated, for each aerial. A very suitable amplifier for the lower radio frequencies is the well-known cathode follower. At higher frequencies, when the cathode follower is not effective, then the grounded-grid amplifier may be used. In the latter case, the grid is grounded for radio frequency potentials, but may be pulsed for aerial switching.

The preferred form of ultra high-frequency switching is, however, a triode valve which is arranged, not as an amplifier, but simply as a Variable impedance in series with the transmitter feed to the aerial. This triode (or in some cases, pair of trlodes) has its anode and grid strapped together for R. F. potentials. and the cathode to anode impedance is used very Vmuch as the cathode to anode impedance of the diode. One advantage of this system is that only a small pulse power is required for impedance modulation.

Another frequent difference between beacon and direction finding technique is the following (though direction finding systems could in some cases use the technique with advantage). In cases where the beacon (or direction finder) is to. serve only part of space, then it is desirable to concentrate transmitted energy into that space. Individual aerials of the beacon (or D. F.) may with advantage be designed to have the requisite directive energy distribution pattern. The individual directive patterns, should, of course, be of very similar shape and strength, in order that unwanted amplitude modulation of the signal shall not be caused.

Still another differentiating aspect of the beacon is that it is often required to radiate intelligence other than bearing information. All of the beacons to :be described hereinafter may radiate speech or other intelligence by simple amplitude or frequency modulation of the whole transmission. In the case of amplitude modulation of the transmitter for example by speech, this is demodulated before subjection to the receiver limiter, where the speech modulation is removed before directional information is determined. l i

Figure 5 showsv diagrammatically `a simple beacon giving a single azimuth course line and embodying `the present invention. In Figure 5 a transmitter, represented by block 25 is used to excite two aerials TAi and TAZ in turn by means of a commutating arrangement as hereinbefore described and represented by block 26 and detectors TDI and TD2. The wave received by a receiver fed continuously from a single antenna system is equivalent to that which would be received by the equivalent D. F. system namely, the homing device hereinbefore described. In order that it shall not be necessary to radiate a separate "sense signal according to a feature of the present invention the two aerials, are excited for unequal periods of time. For this purpose, the commutating arrangement may comprise a pulse generator of rectangular wave form of unequal on and o'if" periods, the output waves therefrom being applied in reverse phases to the detectors TDI andTDZ respectively.

In all directions the receivedwave is substantially unmodulated in amplitude, but undergoes rhythmic phase transients. The phase modulation wave-form is, in fact, in the form of dot or dash according to sense of deviation from the course line, as will be made clear hereinafter.

A receiver for use with the beacon may com,

prise, as shown in Figure 6, a single receiving antenna All) of any desired Suitable form, a high frequency amplifier represented by block 21, an amplitude limiter, represented by block 28, a phaseor frequency modulation demodulator of normal ltype used for exampe in frequency modul lation broadcast systems, and represented by by curveb may be integrated to provide the wave form shown in curve (c) Figure 7 `whichis applied to a centre zero indicating 4instrument of known type usually employed on 'receivers of complementary conjugate dot-dash approach path systems. However, as will be seen hereinafter, more .than two aerials may be .moreedciently employed, in which vcase the phase modulating wave form of the signal would be as shown in curve (d) and output wave from the discriminator 29 .would Vappear somewhat as shown in curve e, Figure 7. This form would be suitable for application to a balanced square law detectortoyield positive or negative D. C. output which is applied to an indicator. This arrangement is represented byblock 3E inFigure 6. Whenmeter indication of the courseis required, the .dotand dashsignals represented in curve-c, Figure 7 (dot signals) .may be converted to positive and negative :direct currents according to any known method usedin blindapproach systems utilising overlapping beams with complementary conjugate dot .dash.signals, for example as described in,U.-S. Patent No.2,431,317, dated November 25, 1947. Such arrangements maybe representedby blocki whichgenerally represents the indicator circuit -arrangement.

When an amplitude modulation speech channel is provided a por-tion of the output from the high frequency or intermediateirequency amplifier Z'Iis `fed to the amplitude modulation demodulator represented by block `3 I, the output of which is fed to a translation device represented by head-phones 32.

Itmay be noted thatthe normal frequency discriminator gives maximum output when phase modul-ation transients equal 90. If the two aerials are spaced at then the course line appears at right angles to the line of aerials, and a meter-operated course meter gives an output which increases progressively accordingto deviation from the course line right up to the end-on position.

As the normal discriminator gives zero response for aphase transient of 1'80 degrees, a false course line appears for antenna spacing greater than so that `Ithe best accentuation -of a single course line without false courses is provided by antenna spacing just short of lFor this simple 2-antenna beacon, aerials may be keyed by separate amplifiers or other electronic keying device, the keying or modulating waves being provided by a simple multi-vibratoror unequal on-oiic ratio as already stated.

'The course line is more easily detected vby the normal frequency modulation discriminator receiver when a third aerial is added at the transmitter as shown in broken line in Fig. -5. For example, if three aerials are arranged in line, TAI, TAE', TA3, making then the phase modulation pattern is according 12 toFigure :7 (d), andfoutput iromlthe frequency; discriminator 2 Q,`Fig..16,. isas :shown in curvef (-e) Fig. 7, for dash signals. The reversed curve would be obtained for dot signals, when the beacon aerials "TAI, TAZ, 4TA3 are excited for equal intervals of time in the sequence TA1 TAZ, TAB, TA1, TA2, TA, etc. Dierentiation oflthe Iphase modulation -by the frequency discriminator 29 gives the wave-form-of curvee,1Fig-. 7. Subjection of this wave e to a course meter circuit in the form of a square-law bridge-rectifieror to a biassed bridge rectifier gives :a negative'D. C. output, owing to the accentuated negative-peak. The inverted partner wave curve (e) as received on the other side of the courseline gives a--corresponding positive D. C. output.

Vligur'e y8 shows by .way ofexamplefthe transmitter beacon arrangementior .a single course line of high precision embodying thepresent invention.

The transmitting antenna system comprises a plurality, greater than two, and five infact being shown, of aerials TAII-'IAleach having connected in series therewith .a respective switching rectifier TDI I-TDE 5. -Each of the .aerialsis connected through its respective switching impedance TDI I -TDI5 to a transmitter equipment represented by block 33 through `21,-respective Yphase adjusting network PNI1I-PNI'5. These latter are so adjusted that the energy fed from v33 is in phase at all the laerials.

The aerials are arranged in line broadside to the required course line. `The aerials are excited in turn, rst from .TAI2 to TAI5 (i. e. v.left'to right) or vice versa, forequal .small intervals of time and then `in .the reverse direction, from TAI@ to TAII or vice versa, for largerintervals of time. To this end, two commutating arrangements are provided .and maybe as described in connection with Figure 2 or 3., using the delay network or series of Kipp relays. .In either case thev commutating devices are represented in Figure 8 by the blocks 34 and 35.

In the .arrangement illustrated Kipp relays represented by blocks 3.6 and 37 are arranged to produce, on operation pulses of substantially square waveform, the pulse from .3l being .of longer duration than the pulse from 36 asfdepicted at 38 and39 respectively.

It Ashould be observed that aerial TAI5is controlledby a pulse from Acommutator.3.1i only and aerial TAI I is controlled by a pulse :from commutator 35 only, .but aerials TAI2, TAIS and TAM, that is the intermediate aerials, are controlled by pulses from both commutators 34 and 35 in proper sequence.

When 34 and .35 vrepresent delay networks, a pulse delayed by a .time .interval equal to the duration of pulse 'il is obtained Ifrom 34 and appliedto triggers-the Kipprelayl whose output pulse 3B is of longer 'duration than :3;9 and is :applied to the input of delay network 35 and .also to control aerial TA |14.. Pu-lses are :obtained from suitable tapping points '1?3, .'I'PEand TPI such r that A`the interval between vthe pulses is equalto the duration of the pulses. These pulses are applied to control the respective Iaerials TAI3, TAI2 and TAI l. A pulse obtained from tap-ping point TPE) on 35 is lsuch that the pulseobtained thereat is delayed behind the pulse obtained from TPrI by Aan interval equal to the pulse 3B duration, is applied to trigger the `Kipp relay 36 which produces the .shorter pulses 39. These pulses are applied to the input of delay network 34 and directly to control-aerial TAI2. Pulses delayed by time intervals equal to the duration of pulse 39 are obtained from tapping points TK3, TK4 and TK5 for controlling aerials TAI3, TAI 4 and TAI 5. A pulse from TKB on delay network 34 is applied to trigger Kipp relay 3l and the action of the commutator repeats the cycle as described.

When blocks 34 and 35 represent series of Kipp relays, the output from the last relay of 3Q is applied to trigger the first relay of series 35, and the output of this first relay is -applied to control TAM. The outputs from the second, third and fourth relays are applied to control respectively TAI3, TAI2 and TAH. The output from the fourth relay of 35 is applied to trigger the rst relay of series 34 whose output is applied to control TAI2. Likewise the outputs of the second, third and fourth relays of 34 control respectively TAI3, TAM and TAIE. The output from the fourth relay of 34 is applied to trigger the rst of relay 35 and the cycle described is repeated.

The receiver employed with a beacon as described with reference to Figure 8, 'will be the same as the receiver described with reference to Figure 6.

Adjacent aerial spacing must not exceed and is preferably not much greater than When aerials are excited from a common continuous wave source, via equal paths from the source, no modulation appears in a direction at right angles to the array.

Oi course, the received signal is modulated in phase according to curve a, Fig. 9, or the same wave-shape inverted in sense, according to sense l of displacement from the course line.

In a practical case of many steps (i. e. Lmany aerials), the phase modulation curve obtained in a receiver of restricted band width will follow the smoother curve b, Fig. 9, 'and differentiation by the frequency demodulator in 29, Fig. 6, yields the Wave-shape of curve c, Figure 9, i. e. dashes for one side of course line, and dots for the other, and may be applied directly to indicatorv circuit 30.

Alternatively, for fewer aerials, and using a moderately broad-band receiver, and broad-band frequency demodulator, the phase modulation curve, curve a, Figure 9, will produce in the output of the discriminator 29 the wave form (0l)l Figure 9, which after slight integration produces the wave form (c) Figure 9. The integrating circuit `may in this instance be a low pass filter.

The arrangement described in connection with Figure 8 also provides a practical form of glidepath beacon utilising an array of horizontal radiatorsl,y each placed very close to the ground (say at As, however, it is not desired to set up a vertical course line, the feeder lengths from transmitter to the various aerials progressively increase from one end of the array to the other in which case the phase adjusters PNI I-PNl may be omitted, or alternately may be used to Iadjust the path. Zero phase modulation is produced in space over the surface of an acute angled cone Whose horizontal axis is the line of the aerial array.

This conical surface can, of course, be 'Inadeto intersect a vertical plane dened by an azimuth system, hence deiining a pre-determined `single glide-path. J

In the case of a glide-path beacon, it is not entirely necessary to avoid false courses at angles Very far removed from the CorrectV course, as owing to excessive steepness these false courses could notbe mistaken by the pilot of an aircraft. Hence, antenna spacing is not limited to a'miximum of Y 2 and the total number of antennae necessary` for a good accentuation of course is not large. f

An omnidirectional beacon embodying the invention is illustrated in Figure 10 and the receiver for using the beacon for determining bearing is illustrated in Figure 1l.

The beacon illustrated in Figure l0 is very similar to the equivalent omnidirectional auto--l matic direction nder illustrated in Figure land it is considered not necessary to give considerable detail.

A plurality of aerials, eight shown in Figure l0, TAH-TAZB are arranged at uniform intervals on the circumference of a circle and are commutated or keyed in rotary succession for equal 4intervals of time, as in the commutator arrangement de,-A scribed in connection` with Figure 2 or 3. I.Such arrangement is represented in Figure yl0 .by the block 40, from which phased pulses of rectangular wave form are applied to respective keying rectifiers e. g. diodes D2! to D28, connected in 'series with respective aerials, to allow the passage oi energy in cyclical successionv from a transmitter represented by the block 4l. As will be observed from Figure 10 the transmitter l is connected to each aerial by the same lengthl of transmission line and all the aerials arethus fed with cophasal energy.

The wavesvfrom the beacon of FigureV l0l received at any point in space is thus Vfrequency: modulated at the commutation rotation frequency of the beacon, and the phase ofthe modulation varies linearly according to the bearing of the receiver lwith respect to the beacon; In order to obtain a direct reading of the bearing it will be necessary to transmit from the beacon some reference signal from which a comparison Wave may be obtained at the receiver. This ma-y. be Vattained by any of the known methods, forl example 1. Synchronising pulses may be transmittedl from the beacon either as positive'pulse of increased radiation, or a negative pulse of momentary reduction or interruption of transmission, whenthe transmitter in a specified geographical location is excited.

2. Amplitude or phase modulation of the transmitted carrier wave from the beacon at half the rotary commutation frequency of the beacon. A comparison Wave of correct frequency may 'be obtained at the receiver by doubling the amplitude or phase-modulation frequency received.

y3. A separate carrier Wave may be 'radiated from the beacon and modulated in any manner by a -comparison wave which can be obtained in known manner at the receiver. This separate carrier may also be used to heterodyne the` main beacontransmission in the receiver, hence converting the received waves to a frequency which enables a very stable frequency demodulator to be used.

A form of receiver for use with the beacon of Figure A10 is shown in Figure l1. In `this ligure the receiving antenna is indicated at AH and as in the case of Figure 1, the output from AH is fed to a high frequency amplifier represented by block 42, an amplitude limiter represented `by block; 43, phase discriminator represented by block ifi, filter represented by block d5, rand the phase comparing circuit and indicator repre* sented by block 46. A circuit arrangement for producing a comparison wave depending upon the method used as hereinbefore referred to is represented by block 41 and the output of lil is fed to the phase comparing arrangement 46 in well-known manner.

If a speech or other signal channel is super imposed on the beacon transmission as an amplitude modulation, an amplitude demodulator .may be fed with a portion of the output from the high frequency amplifying stage v42 as in the case of Figure 6. In the case of a frequency or phase modulation the signal frequencies may be filtered out from the output of the discriminator 4A in known manner.

It will be observed that the output from the discriminator Mi will have a wave form similar to curve (a), Figure 9, except that the steps are equal in the amplitude increasing and decreasing direction. The output of the Yfilter D is a smooth substantially sinusoidal wave.

Whilst several embodiments of the invention have been described it will be understood that details 'in each one may be modified without vdeparting from the spirit and scope of the inven tion.

What is claimed is:

l. A radio blind approach system comprising a transmitter beacon having an array of at least three aerials in alignment arranged broadside to the desired approach path, means for energising said aerials successively first in one direction Vfor equal periods of time and then in the reverse direction for equal longer periods of time, and a receiver for receiving the waves radiated by said beacon, means for amplitude limiting said received waves, a phase or frequency discriminator, and an indicator circuit and means for .applying said limited waves through said discrimi-nator to said indicator circuit.

2. A radio glide-path system comprising a transmitting beacon having an array of at least three aerials in alignment parallel to the ground, means for energising said aerials with appropriately phased energies successively rst in one direction for equal periods of time and then in the reverse direction for equal longer periods of time, and a receiver for receiving the waves radiated by said beacon including an amplitude limiter, a phase or frequency discriininator and an indicator circuit to which the .output of the discriminator circuit is applied.

3. A radio navigational system as claimed in claim 2 wherein said indicator circuit comprises a centre zero indicating instrument.

4. A radio navigational system as claimed in claim 2 wherein said means for energising said aerials comprises two commutator switching arrangements, one designed to provide phased pulses having va duration equal to said irst mentioned equal periods and the other designed to provide phased pulses having a duration equal to said longer periods, means for applying the 16 phased pulses ,to respective vdevices toconnect said aerials to the Vbeacon transmitter cyclically and successively in vforward and lreverse directions, and means for applying the last pulse of one commutator arrangement to start the other commutator arrangement Viii-operation.

5. A direction finding or bearing determining system comprising an omnidirectional beacon having at least three aerials arranged equidis- Icantly around the circumference of a circle, and means for feeding said aerials with energy for transmission for equal short periods of time cyclically and successively, means for transmitting a signal when the cycle of Aaerial feeding passes through a predetermined phase, means for receiving said transmitted energy and signal, means yfor amplitude limiting and phase detecting said received energy, a filter to which the output of said means for discriminating is applied, means for deriving from said received signal, a comparison wave, a phase comparing circuit comprising an indicator and means for applying to said phase comparing circuit the said comparison wave and the output from said filter.

6. A system as claimed in claim 5 wherein said commutator arrangement comprises a plurality of one way detector devices each connected in series with respective aerials between said aerials and the transmitter, means for generating a train of pulses of rectangular wave form whose durations are equal to the period desired for connection of said aerials to the transmitter and having a repetition frequency equal to the commutation cycle, means for producing from said train other pulse trains time phased with respect to each other, and means for applying said time phased trains to respective ones of said detector devices to render these latter conducting cyclically and successively to make connection between said aerials and the transmitter for said period.

7. A system as claimed in claim 5 wherein said commutator arrangement comprises a plurality of one way detector devices each connected in series with respective aerials between said aerials and the transmitter, a series of Kipp relays connected in cascade so that the output of one triggers the next in succession and the output of the last triggers the rst, means for applying the output pulses obtained from the relays to respective one way detector devices .to render these Ilatter conductive cyclically and successively and connect said aerials to the transmittel', said Kipp `relays being so designed as to produce output pulses equal to the period desired' for connection of said device.

aerials to the Vtranslating CHARLES WILLIAM EARP. CHARLES ERIC STRONG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

